Recent research efforts on macroelectronics promise revolutionary applications such as transparent, flexible flat panel displays, solar cells, and large area sensor arrays. The emerging materials to achieve these functions include organic semiconductors, carbon-based semiconductors, and various inorganic nanomaterials. However, metal oxide electronics hold perhaps the greatest promise, with demonstrated large-area compatibility and impressive device performance (electron mobility up to ˜100 cm2/Vs) compared to the current dominant hydrogenated amorphous silicon (a-Si:H) technology (electron mobility ˜1 cm2/Vs). These promising properties have triggered efforts to fabricate oxide electronics on flexible plastic substrates compatible with low-cost, high-throughput solution-processing.
Despite its promise, solution processing of metal oxide electronics typically requires high annealing temperatures (e.g., Tanneal≧400° C.). For example, in conventional sol-gel approaches, a metal precursor (e.g., a metal salt) is used in combination with a base (typically a Bronsted base as a catalyst) and an organic stabilizer (e.g., a ligand compound) to synthesize a sol solution, which is then spun on the substrate. The metal precursor, typically metal alkoxides or metal chlorides, undergoes hydrolysis and polycondensation reactions to form a gel. Formation of the metal oxide involves connecting the metal centers with oxo (M-O-M) bridges, therefore generating metal-oxo and, for partial connections, metal-hydroxo (M-OH•••HO-M) polymers in solution. To densify the metal oxide, that is, to convert the (M-OH•••HO-M) to (M-O-M) lattice by eliminating H2O and to completely burn off the organic portion of the metal oxide film, sintering at high temperatures (≧400° C.) is necessary. As such, current solution-phase techniques employed to process metal oxide films generally are incompatible with inexpensive flexible plastic substrates. The limitations posed by these high processing temperatures have prevented oxide materials from being implemented in large-area flexible macroelectronics.
Furthermore, when metal oxide thin films are derived from pre-formed nanomaterial solutions, an organic ligand similarly is needed to stabilize the solution. Thus after spin-coating, sintering of the resulting nanomaterial films at high temperatures (e.g., >400° C.) again is necessary to remove the ligand. Otherwise, the residual ligands within the film can lead to poor morphological and electrical connections between the nanomaterials, hindering charge carrier transport and limiting the corresponding device performance.
Accordingly, there is a need in the art for new precursor systems and solution-phase processes that can be used to fabricate electronic metal oxide thin films at low temperatures.